Annotation Type Required

This annotation will mark the field as not nullable. When the field is Required,
it cannot be set to null.

Only Boolean, Byte, Short, Integer, Long, Float, Double, String, byte[], Date can be annotated
with Required. Compiling will fail when fields with other types have Required annotation.
Fields with primitive types and the RealmList type are required implicitly.
Fields with RealmObject type are always nullable.